The present invention relates to a method and apparatus for measuring or estimating the three-dimensional profile of a specimen such as a semiconductor wafer in a semiconductor manufacturing process based on images obtained by observing the specimen using an SEM (Scanning Electron Microscope) when the specimen is observed or measured.
With the miniaturization of semiconductor devices, the pre-process for manufacturing these devices is becoming increasingly difficult to control. The electrical characteristics of a semiconductor device pattern are very much affected by its height and line width and the inclination angles of the sidewalls, as well as minute pattern shape variations such as the roundness of the corners. As such, there has been a need to accurately measure these dimensions and shapes to detect process variations and thereby control the process. Thus, effectively controlling the manufacturing process of a semiconductor device requires techniques for observing the sidewalls of the semiconductor device pattern by use of an SEM and estimating its three-dimensional profile based on images obtained by the SEM. Information in relation with SEM images of the specimen as observed obliquely is useful in the above sidewall observation and estimation of the three-dimensional profile.
There are three methods of obtaining an SEM image of a specimen as observed obliquely. One is to deflect the electron beam emitted from a particle source by a pre-lens deflection unit and to redirect the deflected electron beam to the target with a tilt angle by the combined action of an objective lens and an in-lens deflection unit so as to obtain a tilt image of the target (see, e.g., Japanese Patent Laid-Open No. 2000-348658.) Another method is to tilt the stage itself to obtain a tilt image, which is used to move the semiconductor wafer so as to be able to observe any points on the wafer. The other method is to mechanically tilt the SEM electron optical system itself to obtain a tilt image.
Japanese Patent Laid-Open No. 2000-146558 discloses a method for determining gradient on surface to which images are formed with electron beam from a scanning electron microscope (SEM). This method comprises the steps of: processing a calibration procedure which includes the step of obtaining a plurality of calibration electron emission measurements by measuring secondary electron emission from surface portions positioned at respective known different angles θ to the electron beam and the step of deriving a relationship between the plurality of calibration secondary electron emission measurements and the known angles θ; and processing a gradient determination procedure which includes the step of measuring secondary electron emission obtained from each scanning point on the surface of the sample by scanning the electron beam along the surface of the sample and the step of determining the gradient at each scanning point of the surface by deriving angles θ based on the measured secondary electron emissions and the calibration relationship between the plurality of calibration secondary electron emission measurements and the known angles θ.
Conventionally, as described above, an inclination angle (or slope) at each point of the surface is presumed by using by a certain function such as 1/cos(θ), or by using a data table (indicating a relationship between the inclination angle and the amount of secondary electron signal) obtained based on measurements on actual samples as the relationship between the inclination angle and the amount of secondary electron signal. Then, the height information of the surface is obtained by integrating the presumed inclination angle value at each point along the surface. However, since the sensitivity to the inclination angle in the amount of secondary electron signal varies with the inclination angle, it has been difficult to accurately estimate the height information on a flat portion or a nearly vertical portion.